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United States Patent |
5,244,513
|
Kadoya
,   et al.
|
September 14, 1993
|
Fe-Cr-Ni-Si shape memory alloys with excellent stress corrosion cracking
resistance
Abstract
Ferrous group shape memory alloys consisting essentially of Cr: 16.0-21.0
wt %, Si: 3.0-7.0 wt % and Ni: 11.0-21.0 wt % and satisfying Ni wt
%.gtoreq.[0.67.times.{Cr+1.2.times.(Si+Ti+Zr+Hf+V+Nb+Ta)}-] wt % and
(Cr+Si) wt %.gtoreq.20 wt %, these ferrous-group shape-memory alloys
having a corrosion resistance, a shape-memorizing properties, an
intergranular corrosion resistance and a stress corrosion cracking
resistance in nitric acid for nuclear fuel reprocessing plants and
high-temperature, high-pressure water for light-water reactors.
Inventors:
|
Kadoya; Yoshikuni (Takasago, JP);
Yonezawa; Toshio (Takasago, JP);
Ito; Naotake (Kobe, JP);
Inazumi; Toru (Tokyo, JP);
Moriya; Yutaka (Tokyo, JP);
Suzuki; Haruo (Tokyo, JP);
Masamura; Katsumi (Tokyo, JP);
Yamada; Takemi (Tokyo, JP)
|
Assignee:
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Mitsubishi Jukogyo Kabushiki Kaisha (Tokyo, JP);
NKK Corporation (Tokyo, JP)
|
Appl. No.:
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858553 |
Filed:
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March 27, 1992 |
Foreign Application Priority Data
| Mar 29, 1991[JP] | 3-89206 |
| Feb 28, 1992[JP] | 4-078502 |
Current U.S. Class: |
148/402; 420/50; 420/51 |
Intern'l Class: |
C22C 038/46; C22C 038/48; C22C 038/50 |
Field of Search: |
148/402,563
420/50,51
|
References Cited
Foreign Patent Documents |
0336175 | Oct., 1989 | EP.
| |
59-70751 | Apr., 1984 | JP | 148/402.
|
62-112720 | May., 1987 | JP | 148/563.
|
2-077554 | Mar., 1990 | JP.
| |
2-301514 | Dec., 1990 | JP.
| |
Other References
C. W. Wegst, "Stahlschlussel, 15, Auglage". Verlan Stahlschlussel Wegst
GmbH, Title page and pp. 329, 336, 372, and 408 (1989).
H. Funakubo (editor), "Shape Memory Alloys", Sangyo Tosho, Jun. 7, 1984
`Applications of Shape Memory Alloys` by U. Suzuki, pp. 113-114 and 8
(Table 1).
B. E. Wilde, "Corrosion", National Association of Corrosion Engineers
(NACE).
Disclosure Bulletin, vol. 42, No. 11, Nov. 1986, pp. 678-681.
Patent Abstract of Japan, 125 C 726, Jun. 8, 1990 (corres above JP,A554).
Patent Abstract of Japn, 146 C 809, Feb. 21, 1991 (corres above JP,A 514).
|
Primary Examiner: Yee; Deborah
Attorney, Agent or Firm: Frishauf, Holtz, Goodman & Woodward
Claims
We claim:
1. Fe-Cr-Ni-Si shape memory alloys with excellent intergranular corrosion
resistance and stress corrosion cracking resistance, consisting
essentially of Cr: 16.0-21.0 wt %, Si: 3.0-7.0 wt % and Ni: 11.0-21.0 wt %
and any one or two or more of Ti: 0.01-1.0 wt %, Zr: 0.01-2.0 wt %, Hf:
0.01-2.0 wt %, V: 0.01-1.0 wt %, Nb: 0.01-2.0 wt % and Ta: 0.01-2.0 wt %,
satisfying Ni wt %.gtoreq.{0.67(Cr+1.2(Si+Ti+Zr+Hf+V+Nb+Ta))-3} wt % and
0.02 wt %.ltoreq.{Ti+V+0.5(Zr+Nb)+0.25(Hf+Ta)} wt %.ltoreq.2.0 wt % and
having a residue of Fe and inevitable impurities.
2. Fe-Cr-Ni-Si shape memory alloys with excellent intergranular corrosion
resistance and stress corrosion cracking resistance, consisting
essentially of Cr: 16.0-21.0 wt %, Si: 3.0-7.0 wt % and Ni: 11.0-21.0 wt
%, any one or two or more of Mn: 0.1-5.0 wt %, Cu: 0.1-1.0 wt %, N:
0.001-0.100 wt %, Mo: 0.1-3.0 wt % and W: 0.1-3.0 wt %, any one or two or
more of Ti: 0.01-1.0 wt %, Zr: 0.01-2.0 wt %, Hf: 0.01-2.0 wt %, V:
0.01-1.0 wt %, Nb: 0.01-2.0 wt % and Ta: 0.01-2.0 wt %, satisfying
(Ni+0.5Mn+0.06Cu+0.002(C+N)) wt
%.gtoreq.[0.67{Cr+1.2(Si+Ti+Zr+Hf+V+Nb+Ta)+Mo+W}-3] wt % and 0.02 wt
%.ltoreq.{Ti+V+0.5(Zr+Nb)+0.25(Hf+Ta)}wt %.gtoreq.2.0 wt %, and having a
residue of Fe and inevitable impurities.
3. The alloy according to claim 1, which consists essentially of 16.2 to
20.4 wt. % Cr, 3.0 to 6.8 wt. % Si, 14.0 to 20.8 wt. % Ni and any one or
two or more of 0.1 to 0.2 wt. % Ti, 0.2 wt. % Zr, 0.2 wt. % Hf, 0.1 wt. %
V, 0.1 to 0.3 wt. % Nb and 0.3-0.4 wt. % Ta.
4. The alloy according to claim 1, which consists essentially of 16.6 to
20.4 wt. % Cr, 3.0 to 6.8 wt. % Si, 14.0 to 20.8 wt. % Ni and any one or
two or more of 0.1 wt. % Ti, 0.3 to 0.4 wt. % Ta, 0.1 to 0.3 wt. % Nb, 0.2
wt. % Zr and 0.1 wt. % Hf.
5. The alloy according to claim 2, which consists essentially of 16.3 to
19.2 wt. % Cr, 3.8 to 5.0 wt. % Si, 14.1 to 19.0 wt. % Ni, 1.0 to 2.2 wt.
% Mn, 0.4 to 0.8 wt. % Cu, 0.9 to 2.7 wt. % Mo, 0.5 to 2.4 wt. % W, and
any one or two or more of 0.05 to 0.2 wt. % Ti, 0.2 to 0.3 wt. % Nb, 0.2
to 0.4 wt. % Zr and 0.2 wt. % V.
6. The alloy according to claim 1, which consists essentially of 16.2 wt. %
Cr, 5.4 wt. % Si, 15.8 wt % Ni, 0.02 wt % C, 0.01 wt % N and 0.2 wt % Ti.
7. The alloy according to claim 1, which consists essentially of 18.0 wt %
Cr, 4.5 wt % Si, 16.5 wt % Ni, 0.02 wt % C, 0.01 wt % N and 0.3 wt % Nb.
8. The alloy according to claim 1, which consists essentially of 20.4 wt %
Cr, 3.2 wt % Si, 18.7 wt % Ni, 0.01 wt % C, 0.02 wt % N, 0.1 wt % Ti and
0.3 wt % Ta.
9. The alloy according to claim 1, which consists essentially of 16.5 wt %
Cr, 3.6 wt % Si, 12.0 wt % Ni, 0.01 wt % C, 0.01 wt % N and 0.01 wt % V.
10. The alloy according to claim 1, which consists essentially of 16.6 wt %
Cr, 6.8 wt % Se, 20.8 wt % Ni, 0.01 wt % C, 0.01 wt % N and 0.2 wt % Zr.
11. The alloy according to claim 1, which consists essentially of 18.5 wt %
Cr, 5.0 wt % Si, 17.3 wt % Ni, 0.02 wt % C, 0.01 wt % N, 0.02 wt % Zr and
0.2 wt % Hf.
12. The alloy according to claim 1, which consists essentially of 20.1 wt %
Cr, 6.5 wt % Si, 19.7 wt % Ni, 0.02 wt % C, 0.01 wt % N, 0.1 wt % Ti and
0.1 wt % Nb.
13. The alloy according to claim 1, which consists essentially of 18.2 wt %
Cr, 3.0 wt % Si, 14.0 wt % Ni, 0.01 wt % C, 0.01 wt % N and 0.4 wt % Ta.
14. The alloy according to claim 2, which consists essentially of 18.5 wt %
Cr, 4.5 wt % Si, 15.0 wt % Ni, 0.01 wt % C, 0.01 wt % N, 0.1 wt % Ti and
1.0 wt % Mn.
15. The alloy according to claim 2, which consists essentially of 18.4 wt %
Cr, 4.6 wt % Si, 15.2 wt % Ni, 0.01 wt % C, 0.01 wt % N, 0.2 wt % Ti and
2.2 wt % Mn.
16. The alloy according to claim 2, which consists essentially of 18.3 wt %
Cr, 4.4 wt % Si, 15.0 wt % Ni, 0.01 wt % C, 0.01 wt % N, 0.05 wt % Ti, 1.0
wt % Mn and 0.4 wt % Cu.
17. The alloy according to claim 2, which consists essentially of 18.4 wt %
Cr, 4.5 wt % Si, 14.1 wt % Ni, 0.02 wt % C, 0.01 wt % N, 4.1 wt % Mn, 0.8
wt % Cu and 0.3 wt % Nb.
18. The alloy according to claim 2, which consists essentially of 18.0 wt %
Cr, 4.2 wt % Si, 16.8 wt % Ni, 0.02 wt % C, 0.02 wt % N, 1.2 wt % Mo, 0.1
wt % Ti and 0.2 wt % Nb.
19. The alloy according to claim 2, which consists essentially of 16.3 wt %
Cr, 4.2 wt % Si, 18.5 wt % Ni, 0.02 wt % C, 0.02 wt % N, 2.7 wt % Mo, 0.1
wt % Ti and 0.2 wt % Zr.
20. The alloy according to claim 2, which consists essentially of 17.2 wt %
Cr, 3.8 wt % Si, 15.9 wt % Ni, 0.01 wt % C, 0.02 wt % N, 0.9 wt % W and
0.4 wt % Zr.
21. The alloy according to claim 2, which consists essentially of 17.2 wt %
Cr, 4.3 wt % Si, 17.9 wt % Ni, 0.01 wt % C, 0.02 wt % N, 2.4 wt % W and
0.2 wt % V.
22. The alloy according to claim 2, which consists essentially of 16.8 wt %
Cr, 4.5 wt % Si, 15.3 wt % Ni, 0.01 wt % C, 0.01 wt % N, 1.0 wt % Mn, 1.5
wt % Mo and 0.2 wt % Nb.
23. The alloy according to claim 2, which consists essentially of 17.0 wt %
Cr, 4.4 wt % Si, 15.6 wt % Ni, 0.02 wt % C, 0.01 wt % N, 1.0 wt % Mn, 1.0
wt % W and 0.2 wt % Ti.
24. The alloy according to claim 2, which consists essentially of 18.8 wt %
Cr, 3.8 wt % Si, 17.2 wt % Ni, 0.02 wt % C, 0.01 wt % N, 1.2 wt % Mn, 0.9
wt % Mo, 1.3 wt % W and 0.2 wt % Ti.
25. The alloy according to claim 2, which consists of 19.2 wt % Cr, 5.0 wt
% Si, 19.0 wt % Ni, 0.02 wt % C, 0.02 wt % N, 1.9 wt % Mn, 0.05 wt % Cu,
1.8 wt % Mo, 0.5 wt % W and 0.06 wt % Ti.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention concerns Fe-Cr-Ni-Si shape memory alloys with excellent
stress corrosion cracking resistance, and in particular, relates to the
Fe-Cr-Ni-Si shape memory alloys with excellent stress corrosion cracking
resistance, having good shape-memorizing properties, corrosion resistance
and intergranular corrosion resistance in high-temperature, high-pressure
water for the nuclear power field or in nitric acid for nuclear fuel
reprocessing plants.
2. Description of the Prior Art
A ferrous-group shape memory alloy features the property of being restored
to its shape prior to plastic deformation when the alloy is subjected to
plastic deformation at a specified temperature close to the martensite
transformation temperature and then the alloy is heated to a specific
temperature over the inverse transformation temperature to its base phase.
By giving plastic deformation to this shape memory alloy at a specified
temperature, the crystalline structure is transformed from its base phase
into martensite.
As described above, when an alloy which is subjected to plastic deformation
is heated to a specified temperature over the inverse transformation
temperature to its base phase, martensite is inversely transformed to its
original base phase: the alloy shows shape-memorizing properties, by which
the said alloy is restored to its original shape prior to undergoing
plastic deformation. Many non-ferrous shape-memory alloys are already
known as having these shape-memorizing properties. (For example, "Shape
Memory Alloys" edited by Hiroyasu Funakubo, Sangyo Tosho 1984)
Among these conventional non-ferrous shape memory alloys, Ni-Ti and Cu
shape memory alloys are already being put to practical use. Tube joints,
clothing, medical instruments and actuators are manufactured by employing
these non-ferrous shape memory alloys. In recent years, technological
development has progressed to a point where these shape memory alloys are
now applied to a variety of industrial uses.
However, from the viewpoint of application to structural members,
remarkable hydride formation occurs when Ni-Ti shape memory alloys are
used in high-temperature water. Accordingly, they have been unsuitable for
such an environment. Cu-Zn-Al shape memory alloys have insufficient
corrosion resistance. Moreover, these non-ferrous shape memory alloys are
expensive, and from an economical viewpoint, their use is limited.
Under such circumstances, ferrous-group shape memory alloys which are less
expensive than non-ferrous shape memory alloys are being developed. A more
extensive scope of application is envisaged for the ferrous-group shape
memory alloys as opposed to the non-ferrous shape memory alloys which are
restricted in use due to their prohibitive cost. The martensite to which a
ferrous-group shape memory alloy is transformed from its base phase by
undergoing plastic deformation can be roughly divided into fct
(face-centered tetragonal structure), bct (body-centered cubic structure)
and hcp (dense hexagonal structure) from the viewpoint of crystalline
structure. Ferrous group shape memory alloys that are transformed from
their base phase to .epsilon. martensite of dense hexagonal structure by
undergoing plastic deformation and that are excellent in corrosion
resistance are proposed in JP, A No. 2-77554 (hereinafter referred to as
"the first prior art"). That is, the alloys based on the first prior art
contain Cr: 5.0-20.0 wt %, Si: 2.0-8.0 wt %, at least one element selected
in the group comprising Mn: 0.1-14.8 wt %, Ni: 0.1-20.0 wt %, Co: 0.1-30.0
wt %, Cu: 0.1-0.3 wt %, N: 0.001-0.400 wt %, and have excellent
shape-memorizing properties and corrosion resistance.
In JP, A No. 2-301514, alloys containing Cr: 10-17 wt %, Si: 3.0-6.0 wt %,
Mn: 6.0-25.0 wt %, Ni: 7.0 wt % or less, Co: 2.0-10.0 wt % and Ti, Zr, V,
Nb, Mo, Cu, etc. are proposed as high Mn shape memory alloys with a high
Cr content and improved corrosion resistance (hereinafter referred to as
"the second prior art").
On the other hand, B.E. WILDE, "CORROSION-NACE (1986), Vol. 42, No. 11, p.
678" can, for example, be cited as regards ferrous-group alloys with
excellent stress corrosion cracking resistance. That is, this report shows
alloys with excellent stress corrosion cracking resistance in
high-temperature water, containing Cr: 17.0-19.0 wt %, Si: 0.35-4.79 wt %,
Ni: 8.83-9.07 wt %, Mn: 1.30-1.53 wt %, Cu: 0.009-0.20 wt %, N:
0.011-0.040 wt % and Mo: 0.019-0.21 wt % (hereinafter referred to as "the
third prior art").
The shape memory alloys that are used in nitric acid for nuclear fuel
reprocessing plants and in high-temperature water (primary cooling water)
for light-water reactors must have excellent shape-memorizing properties,
intergranular corrosion resistance and stress corrosion cracking
resistance. However, of the said prior arts, none can be found that meets
this requirement.
The ferrous-group shape memory alloys disclosed in the said first prior art
are ferrous-group alloys to which Cr and Si elements are added to improve
the shape-memorizing properties and corrosion resistance and also to which
at least one element of Mn, Ni, Co and N is added. However, these alloys
have the following problems. Though the shape memory alloys show excellent
corrosion resistance, this corrosion resistance was evaluated at an
atmospheric exposure test over a period of two years and the said
intergranular corrosion resistance in nitric acid and stress corrosion
cracking resistance in high-temperature water are not always sufficient.
As seen in its working examples, the basic alloy types can be roughly
divided into the Fe-13Cr-6Si type and the Fe-18Cr-2Si type. The alloys of
the former contain an addition of 15.1 wt % or less of Cr and the alloys
of the latter contain an addition of 2.8 wt % or less of Si. Accordingly,
the improvement in the intergranular corrosion resistance in nitric acid
and stress corrosion cracking resistance in high-temperature water
envisaged as an effect of the addition of Cr and Si is inadequate.
In the first prior art, the C and N content is limited to 0.1 wt % or
less. The study conducted by this inventor and others shows that when
alloys with a total C and N content above 0.01 wt % undergo
thermomechanical treatment indispensable to raise their shape-memorizing
properties (for example, thermomechanical treatment of heating to
500.degree.-700.degree. C. after deformation is given at ambient
temperature), the intergranular corrosion resistance in nitric acid and
the stress corrosion cracking resistance in high-temperature water are
deteriorated by the lack of Cr from the grain boundary due to the
precipitation of Cr carbide or Cr nitride at the grain boundary or the
segregation of C or N at the grain boundary even if the said precipitated
phases do not exist. However, to reduce the total C and N content to 0.1
wt % or less in the alloy composition provided in the same prior art, no
means, except the use of expensive raw materials and/or the use of special
fusions, can be found with existing manufacturing techniques, resulting in
very high cost.
Moreover, in the said first prior art, Co is added as an optional element.
However, as described in the working example, the Co content is 1.0 wt %
or more. Accordingly, the application to high-temperature water (primary
cooling water) in the nuclear power field is unsuitable from the viewpoint
of activation and the applicable scope is limited.
The ferrous-group shape memory alloys disclosed in the second prior art
contain a higher Cr content with the purpose of improving corrosion
resistance and the addition of Ti, Zr, V and Nb, and also high Mn content
with the purpose of raising the shape-memorizing properties. This second
prior art has the following problems. That is, firstly, though the Cr
content is set at 10-17 wt %, the Cr content in the working example is
less than 16 wt %. Accordingly, improvement in the intergranular corrosion
resistance in nitric acid and the stress corrosion cracking resistance in
high-temperature water expected as an effect of the addition of Cr are
inadequate.
Because the Mn content is set at 6.0 wt % or more, the stress corrosion
cracking resistance in high-temperature water is deteriorated by an
increase of non-metal inclusions and the intergranular corrosion
resistance in nitric acid is also deteriorated. Moreover, because the Co
content is set at 2.0 wt % or more, the alloy is unsuitable from the
viewpoint of activation for application to high-temperature water (primary
cooling water) in the nuclear power field and its applicable scope is
limited.
The alloys disclosed in the third prior art show excellent properties of
stress corrosion cracking resistance. However, they can be roughly divided
into alloys with an Si content of 2.9 wt % or less and alloys with an Si
content of 3.8 wt % or more. Regarding the former, the intergranular
corrosion resistance and stress corrosion cracking resistance in the said
environment are inadequate because the Si content is 2.9 wt % or less.
Regarding the latter, the shape memorizing property is inadequate because
the ratio of the total content of austenite-forming elements to the total
content of ferrite-forming elements is not appropriate.
For these reasons, the development of ferrous-group shape memory alloys
with excellent shape-memorizing properties, intergranular corrosion
resistance and stress corrosion cracking resistance that permit their
application to nitric acid for nuclear fuel reprocessing plants and
high-temperature water (primary cooling water) for light-water reactors is
strongly desired. However, such ferrous-group shape memory alloys have not
yet been achieved.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide ferrous-group
shape memory alloys that are free from the technical problems of the said
prior arts.
A more detailed object of this invention is to provide ferrous-group
shape-memory alloys that have excellent shape-memorizing properties,
corrosion resistance and stress corrosion cracking resistance and that are
useable in the high-temperature, high-pressure deionized water (primary
cooling water), typical of the nuclear power field.
It is another object of this invention to provide ferrous-group shape
memory alloys that are excellent in intergranular corrosion and stress
corrosion cracking resistance and that are useable in nitric acid for
nuclear fuel reprocessing plants and in high-temperature, high-pressure
water (primary cooling water) for light-water reactors.
When plastic deformation is given to a ferrous-group shape-memory alloy at
a specified temperature, the phase of the said alloy is transformed from
its base phase of austenite to .epsilon. martensite. After that, when the
said alloy, the base phase of which has been transformed to .epsilon.
martensite, is heated at a temperature above the austenite transformation
temperature (hereinafter referred to as the "Af point") and close to the
Af point, the .epsilon. martensite is inversely transformed to its base
phase of austenite. As a result, the said alloy that underwent plastic
deformation is restored to its original shape prior to the plastic
deformation.
For the ferrous-group shape-memory alloy to have excellent shape-memorizing
properties, the following conditions must be satisfied.
(1) The base phase of the said alloy, prior to undergoing plastic
deformation at a specified temperature, must be composed mainly of
austenite. The said specified temperature means a temperature that permits
transformation from the base phase to .epsilon. martensite when plastic
deformation is given to the said alloy at this temperature.
(2) The stacking fault energy of austenite must be low. In addition, when
plastic deformation is given to the said alloy, the phase of austenite
must be transformed from its base phase only to .epsilon. martensite, and
must not transformed to .alpha.' martensite.
(3) The yield strength of austenite must be high. In addition, when plastic
deformation is given to the said alloy, slide deformation must not occur
in the crystalline structure of the said alloy.
According to this invention, ferrous-group shape-memory alloys that are
useable in nitric acid and high-temperature, high-pressure water and which
particularly show excellent shape-memorizing properties, corrosion
resistance and stress corrosion cracking resistance can be provided by
ferrous-group shape-memory alloys containing Cr: 16.0-21.0 wt %, Si:
3.0-7.0 wt % and Ni: 11.0-21.0 wt %, satisfying Ni wt %.gtoreq.{0.67
(Cr+1.2 Si)-3} wt % and (Cr+Si) wt %.gtoreq.20 wt %, and having a residue
of Fe and inevitable impurities.
Moreover, according to this invention, ferrous-group shape-memory alloys
that are useable in nitric acid and high-temperature, high-pressure water
and which particularly show excellent intergranular corrosion resistance
and stress corrosion cracking resistance can be provided by ferrous-group
shape memory alloys containing Cr: 16.0-21.0 wt %, Si: 3.0-7.0 wt % and
Ni: 11.0-21.0 wt %, with an addition of one or more elements selected from
among Ti: 0.01-1.0 wt %, Zr: 0.01-2.0 wt %, Hf: 0.01-2.0 wt %, V: 0.01-1.0
wt %. Nb: 0.01-2.0 wt % and Ta: 0.01-2.0 wt %, satisfying Ni wt
%.gtoreq.[0.67 {Cr+1.2 (Si+Ti+Zr+Hf+V+Nb+Ta)}-3] wt % and 0.02 wt
%.ltoreq.{Ti+V+0.5 (Zr+Nb)+0.25 (Hf+Ta)}.ltoreq.2.0 wt %, and having a
residue of Fe and inevitable impurities.
DESCRIPTION OF THE DRAWINGS
A complete understanding of the invention may be obtained from the
foregoing and following description thereof, taken in conjunction with the
appended drawings, in which:
FIG. 1 is a graph showing the effect of the Cr and Si content on
shape-memorizing properties of the Fe-Cr-Ni-Si shape-memory alloy
concerned, in a working example of this invention;
FIG. 2 is a graph showing the effect of the Cr and Si content on corrosion
resistance of the Fe-Cr-Ni-Si shape-memory alloy concerned, in a working
example of this invention;
FIG. 3 is an explanatory drawing of the shape of the stress corrosion
cracking test-piece used in a working example of this invention;
FIG. 4 is an explanatory partial cross-section view of the method of
loading stress on the stress cracking test-piece used in a working example
of this invention;
FIG. 5 is a graph showing the effect of the Cr and Si content on stress
corrosion cracking resistance of the Fe-Cr shape memory alloy concerned,
in a working example of this invention;
FIG. 6 shows a plan view of the tensile test-piece prepared in order to
determine the ratio of time to failure from the specimen in a working
example of this invention and in comparative examples;
FIG. 7 is a graph showing the relationship between the ratio of time to
failure obtained from the time to failure in a tensile test in
high-temperature, high-pressure water and a high-temperature atmosphere
with each test-piece shown in FIG. 6, and an amount of Cr and Si content;
FIG. 8 is a graph showing the effect of the Cr and Si content on
shape-memorizing properties of the Fe-Cr-Ni-Si shape memory alloy
concerned, in another working example of this invention;
FIG. 9 is a diagram showing the effect of the Cr and Si content on
intergranular corrosion resistance of the Fe-Cr-Ni-Si alloy concerned, in
another working example of this invention; and
FIG. 10 is a graph showing the effect of the Cr and Si content on stress
corrosion cracking resistance of the Fe-Cr-Ni-Si shape memory alloy
concerned, in another working example of this invention.
In these drawings, the numeral 1 indicates a test-piece, the numeral 2 a
strain gauge, the numeral 3 a holder, and the numeral 4 a clamping bolt.
DESCRIPTION OF PREFERRED EMBODIMENTS
The reason why the chemical composition of the ferrous-group shape-memory
alloys of this invention is limited to the said range is as follows.
Cr acts to reduce the stacking fault energy of austenite and raise the
yield strength of austenite, resulting in an improvement of
shape-memorizing properties. Cr also acts to improve the intergranular
corrosion resistance and stress corrosion cracking resistance of alloys.
With a Cr content below 16.0 wt %, desired results cannot be obtained from
the said actions. For this reason, the lower limit is specified as 16.0 wt
%. On the other hand, if the Cr content exceeds 21.0 wt %, an economic
disadvantage results. Thus, the Cr content should be limited within the
range of 16.0-21.0 wt %.
Si acts to reduce the stacking fault energy of austenite and raise the
yield strength of austenite, resulting in an improvement of
shape-memorizing properties. Si also acts to increase the intergranular
corrosion resistance and stress corrosion cracking resistance. However,
with an Si content below 3.0 wt %, desired results cannot be obtained from
the said actions. On the other hand, when the Si content exceeds 7.0 wt %,
the ductility of the alloy is remarkably lowered, resulting in a marked
deterioration in hot workability and cold workability. Accordingly, the Si
content should be limited within the range of 3.0-7.0 wt %.
Ni is a strong element for the formation of austenite. Ni has the action of
forming the base phase of the alloy prior to plastic deformation into
mainly austenite. If the Ni content is less than 11.0 wt %, the desired
effect of the said action cannot be obtained. Thus, the lower limit is
specified as 11.0 wt %. On the other hand, if the Ni content exceeds 21.0
wt %, the .epsilon. martensite transformation temperature (hereinafter
referred to as the "Ms point") is considerably shifted to the lower
temperature area, thereby lowering the temperature at which the alloy
undergoes plastic deformation and deteriorating the shape-memorizing
properties. Thus, the upper limit is specified as 21.0 wt %. Accordingly,
the Ni content must be limited within the range of 11.0-21.0 wt %.
In this invention, at least one of the following elements can be added in
addition to the said Cr, Si and Ni.
Mn is a strong element for the formation of austenite and has the action of
forming the base phase of the alloy prior to plastic deformation into
mainly austenite. However, if the Mn content is less than 0.1 wt %, this
action cannot be properly attained. On the other hand, if the Mn content
exceeds 5.0 wt %, the intergranular corrosion resistance is deteriorated
and the formation of .sigma. phase is greatly facilitated, thereby leading
to a deterioration in the shape-memorizing properties. So the upper limit
is specified as 5.0 wt %. That is, the Mn content must be limited within
the range of 0.1-5.0 wt %.
Cu is an austenite-forming element, and has the action of forming the base
phase of the alloy prior to plastic deformation into mainly austenite. A
slight addition of Cu has the action of improving the resistance of the
alloy to pitting by corrosion. However, if the Cu content is less than 0.1
wt %, the desired effects of the said actions cannot be obtained. On the
other hand, if the Cu content exceeds 1.0 wt %, the formation of .epsilon.
martensite is checked, thereby deteriorating the shape-memorizing
properties. The reason for this is that Cu acts to raise the stacking
fault energy of austenite. Accordingly, the Cu content should be limited
within the range of 0.1-1.0 wt %.
N is an austenite-forming element, and has the action of forming the base
phase of the alloy prior to plastic deformation into mainly austenite. A
slight addition of N improves the resistance of the alloy to pitting by
corrosion and raises the yield strength of austenite. When the N content
is less than 0.001 wt %, the said actions cannot be properly attained. On
the other hand, if the N content exceeds 0.100 wt %, nitrides of Cr and Si
are easily formed, thereby the shape-memorizing properties of the alloy
are deteriorated. Also, the intergranular corrosion resistance in nitric
acid and the stress corrosion cracking resistance in high-temperature
water are lowered. Even if the Ti, Zr, Hf, V, Nb and Ta to be described
later are added within the range of this invention, satisfactory
improvement cannot be obtained. Accordingly, the N content is limited
within the range of 0.001-0.100 wt %.
Mo is an effective element for improving the intergranular corrosion
resistance and stress corrosion cracking resistance. With a Mo content
below 0.1 wt %, said effects are inadequate. Thus, the lower limit is
specified as 0.1 wt %. However, an addition of more than 3.0 wt %
deteriorates the shape-memorizing properties. Accordingly, the upper limit
is specified as 3.0 wt %.
W is an effective element for improving the intergranular corrosion
resistance and stress corrosion cracking resistance. With a W content
below 0.1 wt %, the effect is inadequate. Also, the addition of more than
3.0 wt % deteriorates the shape-memorizing properties. Accordingly, the
range is specified as 0.1-3.0 wt %. All of Ti, Zr, Hf, V, Nb and Ta are
strong C and N stabilizing elements. By suppressing the precipitation of
Cr carbide or Cr nitride at the crystalline boundary, the effect can be
obtained of checking the deterioration of intergranular corrosion
resistance and stress corrosion cracking resistance. Moreover, the
inventors found that when the total content of C and N exceeds 0.01 wt %,
the intergranular corrosion in nitric acid and the stress corrosion
cracking resistance in high-temperature water were deteriorated by
indispensable thermomechanical treatment to increase the shape-memorizing
properties (for example, thermomechanical treatment of heating to
500.degree.-700.degree. C. after deformation at ambient temperature) even
if the total content of the said elements is low enough (for example, 0.02
wt %) and no precipitation of Cr carbide and Cr nitride is found, and that
addition of C and N stabilizing elements is effective against the
deterioration of these characteristics. To obtain a satisfactory
improvement effect by adding the said elements, the addition of 0.01 wt %
or more of each element is required and
0.02 wt %.ltoreq.{Ti+V+0.5(Zr+Nb)+0.25(Hf+Ta)} wt %.ltoreq.2.0 wt % must be
satisfied. However, if these elements which are ferrite-forming elements
are added copiously, the shape-memorizing properties are deteriorated in
addition to the thermomechanical workability and weldability. Accordingly,
the upper limit for Ti and V is specified as 1.0 wt % and the upper limit
for the other elements is specified as 2.0 wt %.
If substantial amounts of P and S which are impurities are present, the
thermomechanical workability and durability are deteriorated. Thus, the
content of each element must be 0.1 wt % or less. Regarding C being an
impurity, if 0.1 wt % or more is present, the intergranular corrosion
resistance in nitric acid and the stress corrosion cracking resistance in
high-temperature water cannot be satisfactorily improved even if the
aforementioned Ti, Zr, Hf, V, Nb and Ta are added within the range of this
invention. Thus, the C content must be 0.1 wt % or less. Regarding N, its
content must be limited to 0.1 wt % or less for the same reason as C when
it is present as an impurity. Regarding Co being an impurity, a content of
0.1 wt % or less is desirable considering the problem of activation in the
environment of high-temperature deionized water (primary cooling water) of
the nuclear power field.
With regard to the ratio of the total content of austenite-forming elements
to the total content of ferrite-forming elements in this invention, as
described above, the base phase of the alloy prior to subjecting the alloy
to plastic deformation at a specified temperature must absolutely be
composed mainly of austenite. Accordingly, in this invention, the
following expression must be satisfied in addition to the foregoing
limitation for the chemical composition.
Ni wt %.gtoreq.[0.67{Cr+1.2(Si+Ti+Zr+Hf+V+Nb+Ta)}-3] wt %
{Ni+0.5Mn+0.06Cu+0.002(C+N)} wt %
.gtoreq.[0.67{Cr+1.2(Si+Ti+Zr+Hf+V+Nb+Ta)+Mo+W}-3] wt %
That is, by satisfying the above expression, the base phase of the alloy
prior to plastic deformation at a specified temperature can be formed into
mainly austenite. The present invention relates to those alloys that are
excellent not only in shape-memorizing properties, corrosion resistance
and intergranular corrosion resistance but also in stress corrosion
cracking resistance. To attain this excellent stress corrosion cracking
resistance, the following expression for the total content of Cr content
and Si content must be absolutely satisfied in addition to the foregoing
limitation.
(Cr+Si) wt %.gtoreq.20 wt %
That is, by satisfying this expression, excellent stress corrosion cracking
resistance can be attained.
Referring to a concrete working example according to this invention, the
inventors and others melted the steel alloy based on this invention and
the comparative steel alloy of the chemical composition out of the range
of this invention as shown in the following Table 1, in a smelting furnace
under vacuum and cast them into ingot. The ingot obtained in this way were
heated to 1100.degree.-1200 .degree. C. and then hot-rolled to a thickness
of 12 mm. Steel alloy specimens of this invention (hereinafter referred to
as "specimens of this invention") No. 1-19 and comparative steel alloy
specimens out of the range of this invention (hereinafter referred to as
"comparative specimens") No. 20-29 were prepared.
TABLE 1
__________________________________________________________________________
Stress Corrosion Cracking
Shape- Resistance
Alloy Chemical Composition (wt. %)
memorizing
Corrosion Ratio of time
No. Cr Si
Ni Mn Cu N Mo W Property
Resistance
Assessment
to failure
Note
__________________________________________________________________________
Specimens
1 16.0
5.1
15.6
-- -- -- -- --
.circleincircle.
.largecircle.
.largecircle.
0.94
of this
2 18.3
4.8
16.0
-- -- -- -- --
.circleincircle.
.circleincircle.
.largecircle.
1.0
invention
3 20.9
3.0
17.8
-- -- -- -- --
.circleincircle.
.circleincircle.
.largecircle.
1.0
4 20.2
4.6
20.2
-- -- -- -- --
.circleincircle.
.circleincircle.
.largecircle.
1.0
5 16.5
6.8
21.0
-- -- -- -- --
.circleincircle.
.largecircle.
.largecircle.
1.0
6 16.5
3.6
11.2
-- -- -- -- --
.circleincircle.
.largecircle.
.largecircle.
0.91
7 19.5
6.5
20.8
-- -- -- -- --
.circleincircle.
.circleincircle.
.largecircle.
1.0
8 18.2
5.5
18.4
-- -- -- -- --
.circleincircle.
.circleincircle.
.largecircle.
1.0
9 19.0
3.4
17.7
-- -- -- -- --
.circleincircle.
.circleincircle.
.largecircle.
1.0
10
18.1
4.7
15.5
1.1
-- 0.003
-- --
.circleincircle.
.circleincircle.
.largecircle.
1.0
11
18.4
5.0
16.6
0.9
-- 0.005
-- --
.circleincircle.
.circleincircle.
.largecircle.
1.0
12
18.6
5.5
18.2
1.3
-- 0.010
-- --
.circleincircle.
.circleincircle.
.largecircle.
1.0
13
18.4
4.6
16.3
1.0
0.3
0.006
-- --
.circleincircle.
.circleincircle.
.largecircle.
1.0
14
17.0
4.7
15.7
5.0
0.9
-- -- --
.circleincircle.
.largecircle.
.largecircle.
0.97
15
18.5
4.4
16.0
-- -- 0.095
-- --
.circleincircle.
.circleincircle.
.largecircle.
1.0
16
16.4
4.6
18.9
-- -- -- 2.8
--
.circleincircle.
.circleincircle.
.largecircle.
0.95
17
16.6
4.3
19.0
-- -- -- -- 2.7
.circleincircle.
.circleincircle.
.largecircle.
0.94
18
16.2
4.4
18.8
-- -- -- 1.4
1.5
.circleincircle.
.circleincircle.
.largecircle.
0.92
19
16.1
4.5
17.8
1.0
0.3
0.006
1.5
1.6
.circleincircle.
.circleincircle.
.largecircle.
0.93
Comparative
20
15.3
4.0
14.3
-- -- -- -- --
.circleincircle.
X X 0.75
specimens
21
15.1
6.5
17.0
-- -- -- -- --
.circleincircle.
X X 0.80
22
16.2
3.3
15.2
-- -- -- -- --
.circleincircle.
.largecircle.
X 0.85
23
19.2
1.6
13.9
-- -- -- -- --
X .circleincircle.
X 0.88 Impos-
sible to
machine
24
18.1
7.4
20.2
-- -- -- -- --
-- -- --
25
18.1
4.9
16.8
5.8
-- -- -- --
X X .largecircle.
1.0
26
17.9
5.2
15.9
-- 1.5
-- -- --
X .circleincircle.
.largecircle.
1.0
27
18.6
4.4
16.3
-- -- 0.112
-- --
X .circleincircle.
.largecircle.
1.0
28
16.5
4.3
18.0
-- -- -- 3.3
--
X .circleincircle.
.largecircle.
0.95
29
16.5
4.7
18.8
-- -- -- -- 3.2
X .circleincircle.
.largecircle.
0.96
__________________________________________________________________________
Regarding specimens No. 1-19 of this invention and comparative specimens
No. 20-29 obtained in the said way, the shape-memorizing properties,
corrosion resistance and stress corrosion cracking resistance were
investigated in the following tests. The results of these tests are shown
together in Table 1.
(1) Shape memorizing properties
From each of specimens No. 1-19 of this invention and comparative specimens
No. 20-29, a round-shape bar test-piece with a diameter of 6 mm and a
mark-to-mark distance of 30 mm was cut out. A tensile strain of 4% was
added at -196.degree. C. to each test-piece thus formed. Next, each
test-piece was heated to a specified temperature (300.degree. C. or more)
exceeding, but close to, the Af point and the said tensile strain was
added. The mark-to-mark distance of each test-piece after each stage was
measured. According to the result of mark-to-mark measurements, the
shape-recovery rate was calculated by the following expression to evaluate
the shape-memorizing properties of each specimen. (Strictly speaking, the
Ms point slightly differs with specimens. However, the optimum temperature
for plastic deformation was unified to -196.degree. C. for testing.)
The result of the said tensile test is shown in the column "Shape
memorizing properties" of the foregoing Table 1 and the data on specimens
No. 1-9 of this invention and comparative specimens No. 20-23 are shown in
FIG. 1. The criteria for shape-memorizing properties are as follows.
.circleincircle.: Shape recovery rate=70% or more
.largecircle.: Shape recovery rate=30% - below 70%
.times.: Shape recovery rate=Below 30%
##EQU1##
where L.sub.0 : First mark-to-mark distance of test-piece;
L.sub.1 : Mark-to-mark distance of test-piece after addition of tensile
strain; and
L.sub.2 : Mark-to-mark distance of test-piece after heating
In FIG. 1, the abscissa indicates the Si content (wt %) and the ordinate
indicates the Cr content (wt %). In FIG. 1, the range enclosed by the
dotted line shows that the Cr content and Si content are within the range
of this invention. The mark .circleincircle. denotes that the
shape-recovery rate is 70% or more, the mark .largecircle. denotes that
the shape-recovery rate is not less than 30% and less than 70%, the mark
.times. denotes that the shape-recovery rate is less than 30%. As FIG. 1
is self-explanatory, the specimens having a Ni content in the range of
11.0-21.0 wt %, a Cr content within 16.0-21.0 wt % and a Si content within
3.0-7.0 wt %, show excellent shape-recovery properties.
Comparative specimen "23" having 1.6 wt % Si out of the range of this
invention has only very low shape-recovery properties. In FIG. 1,
comparative specimens "20", "21" and "22" out of the range of this
invention resulted in the mark .circleincircle. but are inferior in
corrosion resistance and stress corrosion cracking resistance as described
later.
(2) Corrosion resistance
For each of specimens No. 1-19 of this invention and comparative specimens
No. 20-29, an atmospheric exposure test was executed over a 3 year period
to check their corrosion resistance. After completion of the said test,
the rust occurrence condition of each specimen was visually evaluated.
The criteria for the occurrence of rust are as follows:
.circleincircle.: The occurrence of rust can not be observed.
.largecircle.: The occurrence of rust can be slightly observed.
.times.: The substantial occurrence of rust can be observed.
The results of the said test are shown in the column "Corrosion Resistance"
in Table 1, and the results on specimens No. 1-9 of this invention and
comparative specimens No. 20-23 are also shown in FIG. 2.
As Table 1 and FIG. 2 are self-explanatory, the specimens within the range
of this invention show excellent corrosion resistance. Because comparative
specimen No. 23 out of the range of this invention was inferior in
shape-memorizing properties as shown in Table 1 and FIG. 1, it is not
included in the claims of this invention. Comparative specimen No. 22,
which was inferior in stress corrosion cracking resistance as shown in
Table 1 and FIG. 5, is also not included in the claims of this invention.
(3) Stress corrosion cracking resistance
From each of specimens No. 1-19 of this invention and comparative specimens
No. 20-29, a test-piece shown in FIG. 3 was cut out and each test-piece
thus cut out was set in holder 3 shown in FIG. 4. Next, strain gauge 2 was
fixed on test-piece 1 and clamping bolt 4 was forced in. Strain
corresponding to a specified stress (yield stress) was given, and the
test-piece was dipped under the stress corrosion cracking test conditions
shown in the following Table 2. After a period of 3000 h, the surface of
the test-piece was checked for cracking. Thus, the stress corrosion
cracking resistance of each specimen was evaluated.
TABLE 2
______________________________________
Stress Corrosion Cracking Test Conditions
______________________________________
Temperature 300.degree. C.
Residual oxygen concentration
8 ppm
Cl 0.01 ppm or less
pH (at 25.degree. C.) 7 .+-. 0.2
______________________________________
The results of the said stress corrosion cracking test are shown in the
column "stress corrosion cracking resistance" in Table 1, and results on
specimens No. 1-9 of this invention and comparative specimens No. 20-23
are also shown in FIG. 5.
The criteria for the occurrence of cracking in the said stress corrosion
cracking test are as follows.
.largecircle.: The occurrence of cracking can not be observed. (No
cracking)
X: The occurrence of cracking can be observed. (Occurrence of cracking)
In FIG. 5, the abscissa shows the Si content (wt %) and the ordinate shows
the Cr content (wt %). In this FIG. 5, the range enclosed by the dotted
line shows that the Cr content and Si content are within the range of this
invention. In FIG. 5, the mark ".largecircle." denotes that no cracking
was observed and the mark "X" denotes that cracking was observed.
As FIG. 5 is self-explanatory, the specimens having a Ni content within the
range of 11.0-21.0 wt %, and the specimens having a Cr content within the
range of 16.0-21.0 wt % and a Si content within the range of 3.0-7.0 wt %
show excellent stress corrosion cracking resistance. Conversely,
comparative specimen "20" having a Cr content of 15.3 wt % which is out of
the range of this invention, comparative specimen "21" having a Cr content
of 15.1 wt %, and comparative specimen "22" having a total content of Cr
and Si of 19.5 wt %, namely, less than 20 wt %, and comparative specimen
"23" having a Si content of 1.6 wt % are insufficient in stress corrosion
cracking resistance.
Moreover, the inventors made a study of this stress corrosion cracking
resistance and prepared each 2 mm thick test-piece 5 of a mark-to-mark
distance of 25 mm in the test part 6 with a width of 5 mm between the
holding parts 7 at both ends, shaped as shown in FIG. 6 by using the said
specimens 1-19, 20-23 and 25-29 shown in Table 1.
The tensile test conditions for each said test-piece are as shown in the
following Table 3 and the times to failure obtained by these tests were
evaluated.
TABLE 3
______________________________________
In high-temp,
high-pressure water
In high-temp atmosphere
______________________________________
Test Temp
320.degree. C. 320.degree. C.
Pressure 180 atmospheres
Atmospheric pressure
Atmosphere
water air
(residual oxygen:
8 ppm)
Strain rate
3 .times. 10.sup.-6 /sec
3 .times. 10.sup.-6 /sec
______________________________________
Regarding the obtained results, (time to failure in high-temperature,
high-pressure water)/(time to failure in atmospheric pressure air),
namely, the ratio of time to failure is included in the foregoing Table 1.
The relationship between this ratio of time to failure and the content of
{Cr+Si} (wt %) of each specimen is shown in FIG. 7.
That is, it was confirmed by this invention that when the content of
{Cr+Si} became 20 wt % or more, the said ratio of time to failure became
0.9 or more and the same failure properties as in atmosphere pressure air
were shown in high-temperature, high-pressure water.
Referring to another concrete working example of this invention, the
inventors melted the steel alloy (No. 31-50 based on this invention and
the comparative steel alloy (No. 51-61) with chemical compositions out of
the range of this invention, shown in the following Table 4, in a smelting
furnace under vacuum and cast them into ingot. The ingot thus obtained
were heated to 1100.degree.-1200.degree. C. and then hot rolled to a
thickness of 12 mm to provide specimens. Regarding each specimen, the
shape-memorizing properties, intergranular corrosion resistance and stress
corrosion cracking resistance were checked in the following tests. The
results of these tests are shown together in Table 4.
TABLE 4
__________________________________________________________________________
Inter-
Stress
Shape-
granular
Corrosion
Alloy Chemical Composition (wt. %) memorizing
Corrosion
Cracking
No. Cr Si
Ni Mn Cu
Mo W C N Others Property
Resistance
Resistance
Note
__________________________________________________________________________
Specimens
31
16.2
5.4
15.8 0.02
0.01
0.2Ti .circleincircle.
.largecircle.
.largecircle.
of this
32
18.0
4.5
16.5 0.02
0.01
0.3Nb .circleincircle.
.circleincircle.
.largecircle.
invention
33
20.4
3.2
18.7 0.01
0.02
0.1Ti, 0.3Ta
.circleincircle.
.circleincircle.
.largecircle.
34
16.5
3.6
12.0 0.01
0.01
0.1V .circleincircle.
.largecircle.
.largecircle.
35
16.6
6.8
20.8 0.01
0.01
0.2Zr .circleincircle.
.circleincircle.
.largecircle.
36
18.5
5.0
17.3 0.02
0.01
0.2Zr, 0.2Hf
.circleincircle.
.circleincircle.
.largecircle.
37
20.1
6.5
19.7 0.02
0.01
0.1Ti, 0.1Nb
.circleincircle.
.circleincircle.
.largecircle.
38
18.2
3.0
14.0 0.01
0.01
0.4Ta .circleincircle.
.circleincircle.
.largecircle.
39
18.5
4.5
15.0
1.0 0.01
0.01
0.1Ti .circleincircle.
.circleincircle.
.largecircle.
40
18.4
4.6
15.2
2.2 0.01
0.01
0.2Ti .circleincircle.
.circleincircle.
.largecircle.
41
18.3
4.4
15.0
1.0
0.4 0.01
0.01
0.05Ti .circleincircle.
.circleincircle.
.largecircle.
42
18.4
4.5
14.1
4.1
0.8 0.02
0.01
0.3Nb .circleincircle.
.circleincircle.
.largecircle.
43
18.0
4.2
16.8 1.2 0.02
0.02
0.1Ti, 0.2Nb
.circleincircle.
.circleincircle.
.largecircle.
44
16.3
4.2
18.5 2.7 0.02
0.02
0.1Ti, 0.2Zr
.circleincircle.
.circleincircle.
.largecircle.
45
17.2
3.8
15.9 0.9
0.01
0.02
0.4Zr .circleincircle.
.circleincircle.
.largecircle.
46
17.2
4.3
17.9 2.4
0.01
0.02
0.2V .circleincircle.
.circleincircle.
.largecircle.
47
16.8
4.5
15.3
1.0 1.5 0.01
0.01
0.2Nb .circleincircle.
.circleincircle.
.largecircle.
48
17.0
4.4
15.6
1.0 1.0
0.02
0.01
0.2Ti .circleincircle.
.circleincircle.
.largecircle.
49
18.8
3.8
17.2
1.2 0.9
1.3
0.02
0.01
0.2Ti .circleincircle.
.circleincircle.
.largecircle.
50
19.2
5.0
19.0
1.9
0.5
1.8
0.5
0.02
0.02
0.06Ti .circleincircle.
.circleincircle.
.largecircle.
Comparative
51
15.2
3.5
14.5 0.01
0.01
0.1Ti .circleincircle.
X X
specimens
52
15.0
6.2
17.7 0.01
0.01
0.1Ti, 0.1Nb
.circleincircle.
X X
53
17.1
1.8
13.9 0.01
0.02
0.4Zr X X X
54
18.3
2.6
15.0 0.01
0.01
0.2Nb .largecircle.
.largecircle.
X
55
18.2
4.9
16.8
1.0 0.02
0.02
0.01Ti .circleincircle.
X X
56
18.0
4.5
15.5
1.0
0.3
0.8 0.02
0.01 .circleincircle.
X X
57
20.5
4.6
20.2 0.3
1.5
1.6
0.01
0.02 .circleincircle.
X X
58
18.1
4.7
16.9
5.7
1.5 0.02
0.01
0.2Ti X X X
59
18.6
4.4
15.9
1.0
0.2
3.8 0.01
0.01
0.2Nb X .circleincircle.
.largecircle.
60
18.0
5.9
17.0
1.0 3.5
0.01
0.01
0.2Nb X .circleincircle.
.largecircle.
61
18.0
7.5
17.1
1.0 0.01
0.01
0.1Ti -- -- -- Impos-
sible to
machine
__________________________________________________________________________
Each property in the foregoing Table 4 is as follows:
(1) Shape-memorizing properties
From each of specimen alloys No. 31-50 of this invention and comparative
specimen alloys No. 51-60, a round-shape bar test-piece with a diameter of
6 mm and a mark-to-mark distance of 30 mm was cut out and a tensile strain
of 4% was added at -196.degree. C. to each test-piece thus cut out. Next,
each test-piece was heated to a specified temperature (300.degree. C. or
more) exceeding, but close to, the Af point and the said tensile strain
was added. The mark-to-mark distance of each test-piece after heating was
measured. According to the result of mark-to-mark measurement, the
shape-recovery rate was calculated by the following expression to evaluate
the shape-memorizing properties of each specimen alloy. (Actually, the Ms
point differs with specimen alloys. However, the optimum temperature for
giving plastic deformation was unified to -196.degree. C. for test
purposes.)
The results of the said shape memorizing property tests are shown in the
column "Shape-Memorizing Properties" in the foregoing Table 4 and the
results on alloys No. 31-of of this invention and comparative alloys No.
51-54 is shown in FIG. 8. The criteria used here for shape-memorizing
properties are the same as those for the preceding working example.
In FIG. 8, the abscissa indicates the Si content (wt %) and the ordinate
indicates the Cr content (wt %). In this FIG. 1, the range enclosed by the
dotted line shows that the Cr content and Si content are within the range
of this invention. The evaluation of the shape-recovery rate is the same
as that for the preceding working example in FIG. 1. As FIG. 8 is
self-explanatory, the specimen alloys having an Ni content within the
range of 11.0-21.0 wt %, a Cr content within the range of 16.0-21.0 wt %
and an Si content within the range of 3.0-7.0 wt % show excellent
shape-memorizing properties.
The comparative alloy "53" containing 1.8 wt % Si which is out of the range
of this invention has only very low shape-memorizing properties. The
comparative alloy "54" containing 2.6 wt % Si shows shape-memorizing
properties belonging to this mark ".largecircle." but is inferior to the
evaluation of the alloys of the invention and also inferior in the
intergranular corrosion resistance and stress corrosion cracking
resistance as described later. In FIG. 8, the comparative alloys "51" and
"52" which are out of the range of this invention also show sufficient
shape-memorizing properties but are inferior in intergranular corrosion
resistance and stress corrosion cracking resistance as described later.
(2) Intergranular corrosion resistance
From each of alloys No. 31-50 of this invention and comparative alloys No.
61-60, a plate-shaped test-piece with a thickness of 4 mm, a width of 20
mm and a length of 100 mm was cut out. A tensile strain of 4% was given at
ambient temperature and then heated to 600.degree. C. This was repeated 3
times. After that, a plate-shaped test-piece for corrosion testing with a
thickness of 2 mm, a width of 15 mm and a length of 20 mm was cut out from
each said plate-shaped test-piece. This test-piece was dipped in boiled
40% nitric acid after its surface was wet-polished up to #600. After
5-days' dipping, the cross-section of the test-piece was observed with an
optical microscope and the maximum intergranular corrosion depth was
checked to evaluate the intergranular corrosion resistance. The results of
the intergranular corrosion test are shown in the column "Intergranular
corrosion resistance" in the foregoing Table 4 and the data on alloys No.
31-38 of this invention and comparative alloys No. 51-54 is shown in FIG.
9. The criteria for intergranular corrosion resistance are as follows.
.circleincircle.: The maximum corrosion depth is less than 10 .mu.m;
.largecircle.: The maximum corrosion depth is not less than 10 .mu.m and
not more than 30 .mu.m; and
X: The maximum corrosion depth is not less than 30 .mu.m.
As Table 4 and FIG. 9 are self-explanatory, the alloys having an Ni content
within the range of 11.0-21 wt %, a Cr content within the range of
16.0-21.0 wt % and a Si content within the range of 3.0-7.0 wt % show
excellent intergranular corrosion resistance.
The comparative alloys "51" and "52" having a Cr content of less than 16 wt
% which are out of the range of this invention, the comparative alloy "53"
having a Si content of 1.8 wt %, and the comparative alloys "55", "56" and
"57" with insufficient addition or no addition of C and N stabilizing
elements such as Ti and the comparative alloy "58" with the addition of
more than 5.0 wt % Mn show poor intergranular corrosion resistance. The
comparative alloy "54" with a Si content of 2.6 wt % is evaluated as mark
".largecircle." but is inferior in stress corrosion cracking resistance as
described later.
The comparative alloys "59" and "60" with an addition of 3.0 wt % or more
Mo and W show excellent intergranular corrosion resistance but are
inadequate in shape-memorizing properties.
(3) Stress corrosion cracking resistance
From each of alloys No. 31-50 of this invention and comparative alloys No.
51-60, similar to those of the preceding working example, a plate-shaped
test-piece with a thickness of 4 mm, a width of 20 mm and a length of 100
mm was cut out. Then, a tensile strain of 4% was given at ambient
temperature and then heated to 600.degree. C. This was repeated 3 times.
After that, the test-piece shown in FIG. 3 was cut out from each said
plate-shaped test-piece. Each test-piece thus cut out was set in the
holder 3 shown in FIG. 4. After that, the strain gauge 2 was fixed on
test-piece 1 and clamping bolt 4 was forced in. The strain corresponding
to a specified stress (yield stress) was given, and the test-piece was
dipped under the same stress corrosion cracking test conditions as shown
in the working example in Table 2. After a period of 3000 h, the surface
of the test-piece was checked for cracking. Thus, the stress corrosion
cracking resistance of each alloy was evaluated.
The results of the said stress corrosion cracking test are shown in the
column "Stress corrosion cracking resistance" in Table 4 and the results
on alloys No. 31-38 of this invention and comparative alloys No. 51-54 are
shown in FIG. 10. The criteria for the occurrence of cracking in this
stress corrosion cracking test are the same as for the preceding working
example. FIG. 10 shows the effect of the Cr and Si content on stress
corrosion cracking resistance of Fe-Cr-Ni-Si shape memory alloys in the
working examples of this invention. In this FIG. 10, the abscissa shows
the Si content (wt %) and the ordinate shows the Cr content (wt %). In
this FIG. 10, the range enclosed by the dotted line shows that the Cr and
Si content are within the range of this invention. In FIG. 10, the mark
".largecircle." denotes that no cracking was observed and the mark "x"
denotes that cracking was observed.
As Table 4 and FIG. 10 are self-explanatory, the alloys having an Ni
content within the range of 11.0-21.0 wt %, a Cr content within the range
of 16.0-21.0 wt % and a Si content within the range of 3.0-7.0 wt % show
excellent stress corrosion cracking resistance. On the other hand, the
comparative alloys "51" and "52" containing less than 16.0 wt % Si which
is out of the range of this invention, the comparative alloy "53"
containing 1.8 wt % Si, the comparative alloy "54" containing 2.6 wt % Si,
the comparative alloys "55", "56" and "57" with insufficient addition or
no addition of C and N stabilizing elements such as Ti, and the
comparative alloy "58" with an addition of Mn exceeding 5.0 wt % are
inferior in stress corrosion cracking resistance. The comparative alloys
"59" and "60" with addition of 3.0 wt % or more Mo and W show excellent
stress corrosion cracking resistance but are inadequate in
shape-memorizing properties.
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